\section{Conclusions} % (fold)
\label{sec:Conclusions}

\subsection{Achievements and Validation of Concept} % (fold)
\label{sub:Achievements and Validation of Concept}

A prototype small balloon-lofted attitude stabilisation platform has been designed and partly constructed using mostly commercially available components and construction methods, for a cost of around £1000.

A sensor fusion algorithm using a Kalman Filter to track gyroscope bias was successfully designed and tested, and showed very significantly improvements in estimating angles over simple integration of angular rate gyro output, validating the concept of combining low-cost MEMS sensor with more sophisticated processing over the use of high performance (but much more expensive) inertial sensors with simpler signal processing.

A capable flight computer was designed and built which combined an inertial measurement unit, GPS location, high current input/outputs for PWM and pyrotechnic activation, a radio transceiver for simplex communication, and other peripherals which should allow a great deal of flexibility for further development of this project and in future projects.

Tests flights of the flight computer were performed and it performed to specification, demonstrating and verifying almost all of its capabilities.

A new technique for demodulation of FSK signals using Bayesian Changepoint detection was investigated and shows promising initial results. Developing this technique into mission-reliable online software was not slightly beyond the scope of the project and could not have been achieved within the tight time constraints, but development of this technique will be actively continued and hopefully developed into a flexible PC application over the summer of 2010.

Analysis indicated that a yaw stabilisation system could be constructed with a hybrid reaction wheel and suspension-twist motor to regulate the momentum stored in the reaction wheel.

Although the project has not yet demonstrated a stabilising control system, the underlying hardware and software building blocks function correctly and bench and integration tests will continue and a full test flight will be made as soon as possible.

\subsection{Project Significance} % (fold)
\label{sub:Project Significance}

One of the aims of the project was to investigate the performance limits of MEMS sensors when used with modern estimation techniques. This project has demonstrated a Kalman Filter based technique to track and remove gyroscope bias to create a long-term stable inertial reference. This Filter runs on a cheap commodity 32bit processor. This has applications significantly beyond the scope of this project, especially in the emerging field of micro unmanned aerial vehicles and human-computer interface (the Nintendo Wii controller makes use of accelerometers for example, but does not use Kalman Filtering \cite{wii}).

It is hoped that stabilisation techniques developed for this project can have applications beyond astronomy. Using a small balloon and cheap stabilisation system is perhaps less useful for experiments doing visible and infra-red observations because such flights often require a large collecting mirror, which instantly puts quite a high lower bound of mission size and cost. However, high altitude balloons are increasingly being investigated for use of other applications, such as rapidly deployable long range communications. Currently such systems need lower gain omni-directional antennas to cope with the effects of movement of the balloon payload. The system developed in this project would allow payloads to be stabilised and thus higher gain antennas could be used, such as microwave dishes.

Radio Spectrum in an increasingly valuable resource and there is great regulatory pressure to limit bandwidths and transmission powers on almost all wavelengths. Thus the work done in using Bayes techniques for demodulation to improve the Signal to Noise threshold for a given Bit Error rate has wide applications. The additional computation complexity over conventional techniques becomes less of an issue as transistor density increases, and the algorithm is highly parallelisable, making it a very good candidate for hardware acceleration. 
% subsection Project Significance (end)

\subsection{Evaluation of Approach} % (fold)
\label{sub:Evaluation of Approach}

The project was an ambitious one, combining a lot of practical work with analysis and Bayesian techniques for signal processing. Whilst the construction of a fully working and flight-ready prototype within the time limit was arguably unrealistic, a number of things have been learned over the course of the project:

\begin{itemize}
  \item The construction of the vehicle was not completed and this is because the design and development took significantly more time than was originally envisaged. This was due to over-optimistic estimation of the time required for practical tasks and the author neglected to consider design rework, iteration, software debugging and so on. There is a great deal of truth in the dictum often heard (for which no reliable attribution could be found):

\begin{quote}
	The difference between theory and practice is often smaller in theory than in practice.
\end{quote}

	\item During integration, problems with subsystems were frequently resolved with a methodical, reductionist approach of individually testing the components in isolation. However, there are circumstances in which this is difficult. Writing unit tests for embedded software is difficult because, especially with low-level interface drivers, because they really on fairly complex interactions with other subsystems. A specific example would be debugging the interface with the SD memory card. This was traced to a difficult to characterise problem with SD card 'wait states' where no more data can be written for a period of time, sometimes of the order of 100s of milliseconds. The SD interface is proprietary so documentation is not freely available. It took almost a week to resolve this bug, resulting from a complex interaction of the embedded file system library, the SD-card hardware peripheral on the microcontroller and the SD card itself.

	\item The project was hampered by the vagaries of the English weather. An unusually strong jet-stream throughout the first part of 2010 severely limit the number of days where a balloon launch was viable, and this prevented a balloon test flight of some of the hardware taking place. When it was wished to attempt a test flight of the yaw stabilisation hardware, civil aviation permission to launch could not be obtained because of their administrative load in dealing with the Icelandic Volcano. The author maintains that incremental test flights are good engineering practice, but they do not fit well with the time constrains of a IIB project.
\end{itemize}

\subsection{Future Work} % (fold)
\label{sub:Future Work}

The project will be actively continued over the summer of 2010. It is hoped the following will be achieved:

\begin{itemize}
  \item Completion of hardware manufacture
  \item Thorough bench testing of control system
  \item Test flight with yaw stabilisation
  \item Full test flight with small imaging payload
\end{itemize}

Following on from this, a control enhancement could be made:

\begin{itemize}
	\item A true multi-input-multi-output controller could be designed to replace the decoupled per-axis SISO controllers currently implemented. This would allow account to be taken of effects such as pitch variation changing the moment of inertia of the vehicle about the yaw axis.
\end{itemize}

Further work could also be undertaken to improve the channel model used in the Bayesian Changepoint Demodulation scheme. This would include modelling the channel as a Mixture of Gaussians (MoG) as described in \cite{mackay}. 

% subsection Future W (end)

% subsection Evaluation of Approach (end)



% subsection Achieveorkments and Validation of Concept (end)
% section Conclusions (end)
